Radar techniques can be used to detect minute body movements, including movements associated with cardiac and respiratory activity, body tremors, and gait signature. Functionally, a non-invasive, electromagnetically-based vital signs detection system is an extremely sensitive motion detection system capable of detecting small body motions. Motion detection is achieved by transmitting an interrogating electromagnetic field at the target of interest, and then measuring the time-delay of the return signal reflected back from the surface of the target. When the target surface is moving, as does the surface of the chest in conjunction with respiratory and cardiac activities, corresponding variations will be observed in the measured time delay. The observed variations can be used to determine motion-related target parameters such as displacement and velocity.
An electromagnetically-based vital signs detection system can easily be used in a completely non-contacting mode and can be placed an appreciable distance from the test subject. Electromagnetic signals in the microwave band are also capable of penetrating through heavy clothing and walls.
Applications can include heart and respiration monitoring, heart disease screening, finding persons behind walls and in closed areas, and deception detection (lie detector) using heart and respiration channels of polygraph.
In operation of a vital signs detection system, a beam of radio frequency energy is directed towards the body of a subject. The reflected signal contains phase information representing the movement of the surface of the body, from which respiration and heartbeat information can be obtained. The reflected phase modulated energy is typically received and demodulated by the detection system using synchronous quadrature detection. The quadrature signals so obtained can then be signal processed to obtain the heartbeat and respiratory information of interest. Analysis of the vital signs waveform can be used for estimation of, for example, cardiac function and blood pressure.
One problem in sensing the movement of the surface of a subject is the occurrence of periodic nulls in the sensing field. Thus, it is possible to fail to detect motion from a subject depending on the subject's range from the transceiver. This problem is referred to as a null detection point problem. In a typical technique, two quadrature channels are compared, and then the result from the channel with better performance (i.e., the result closer to the optimal detection point) is used. However, the channel having better performance is dependent on the distance between the transceiver and the subject, which changes from case to case in real applications.
Another technique to avoid a null detection point is using double-sideband transmission and frequency tuning. However, this technique requires tuning the intermediate frequency as the subject-to-antenna distance changes.
A further challenge for non-contact vital sign detection is the noise caused by random body movement, which presents severe interference for accurate detection of respiration and heartbeat signal in practical applications. Since random body movement is comparable or even stronger than certain weak vital sign signals, it has been difficult to implement non-contact vital sensors.
Accordingly, there exists a need in the art for improved detection accuracy of a vital signs waveform.